During the production of printing forms, the printing and non-printing surface parts must be structured in accordance with the information to be printed. Thus, it is known to convert thermally changeable coating applied to a base material into oleophilic, ink-accepting parts by heating the coating using suitable means such as a laser beam. The thermally changeable coating material has hydrophilic properties.
Printing forms work with a printing unit and can be of several types. For example, the forms can be plates clamped onto a cylinder, which are commonly used for off-set printing. Alternatively, they can be films in a special printing form image printing device or a surface of a machine cylinder. In a flat bed printing method using direct image printing, a laser is commonly used to write onto the printing forms. As a result of the input of heat by means of the laser, a thermally activatable coating on the forms has hydrophilic properties. It is converted or transformed from a hydrophilic state to an oleophilic, ink-accepting state in response to heat generated by application of the laser light to a spot-sized area (hereinafter the "image point").
The- temperature applied to an image point on the thermally activatable coating of a form must exceed a specific threshold value. This means that the energy applied to the coating by the laser must locally exceed a threshold value, which in turn generates thermal heat that raises the temperature of the image point above an "image setting temperature." This "image setting temperature" (e.g., 280.degree.) must be exceeded for a predefined period of time such as one to two microseconds. The image point is not set if this temperature versus time characteristic is not observed. This temperature versus time characteristic is predefined and oriented to the chemical and physical properties of the thermally activatable material.
Writing printing forms by means of supplying heat is in this case unable to structure the surface with respect to the information to be printed simply on the basis of a physical and/or chemical change to the coating material. In this context, methods for image printing by means of thermal abrasion are also known in which an image is created based on a point-by-point energy input. Likewise, with a requirement that a limiting temperature be exceeded for a minimum action time, removal of the coating material from the plate occurs, for example, by evaporation or by restructuring of the plate caused by changes to the plate's volume.
In all of the methods described above for using laser radiation as a source of heat energy to print images in a thermally activatable coating material, the material must be heated to exceed a temperature for a predetermined time interval. Depending on the type of the image printing method and on the composition of the thermally activatable coating material, relatively high laser powers must be employed. The lasers are modulated to provide the minimum time period required for heating each image point, which has a negative effect on the maximum achievable speed of writing precisely onto a large-format printing plate. In the case of this thermal image printing method using laser radiation, however, it is not only disadvantageous to need high laser power that has to be modulated, it is also disadvantageous to require a high amount of thermal energy to be applied to the plate. The higher the temperature necessary to set an image in the thermally activatable coating, the greater the temperature difference between each image point and the region of the plate surrounding the point. This large thermal gradient introduces stresses which can, for example, permanently detach the coating and base material or create deformations in at least one of these two layers of materials. It is also possible for a specific type of thermally activatable coating material to have very good thermal writing properties but nevertheless be considered an undesirable image printing material because it requires excessively high temperatures that accentuate the problems of material separation and/or deformation caused by thermal stresses.